1. Field of the Invention
This invention relates to a high sodium ion conducting inorganic composite solid electrolyte and a method for the production thereof, and more particularly to a high sodium ion conducting inorganic composite solid electrolyte for use as a solid electrolyte for a sodium/sulfur storage battery and a method for the production thereof.
2. Description of the Prior Art
The lead-acid battery is the type most popularly used as a secondary battery. Since the sodium/sulfur storage battery has a far larger energy density than the lead storage battery (4.7 times that of the lead storage battery), the development thereof for power storage or as a power source for driving an electric automobile is urgently desired.
Heretofore, the use of .beta.-alumina (Na.sub.2 O11Al.sub.2 O.sub.3) as a solid electrolyte for the sodium/sulfur storage battery has prevailed because this storage battery requires a solid electrolyte with high ion conductivity. The polycrystal of .beta.-alumina exhibits a high conductivity in the range of 3.times.10.sup.-4 -5.times.10.sup.-4 S/cm at room temperature (25.degree. C.). However, it has the disadvantages of offering low chemical stability to water, requiring a high temperature exceeding 1600.degree. C. for synthesis, and being difficult to manufacture as a sinter without cracks.
Thus, the development of a material free of these defects and suitable as the solid electrolyte of the sodium/sulfur storage battery has been desired. Na.sub.1+x Zr.sub.2 P.sub.3-x Si.sub.x O.sub.12 (0.ltoreq.x.ltoreq.3) is counted among the alternatives to .beta.-alumina as a material having sodium ion conductivity. Though this material shows maximum conductivity (10.sup.-3 S/cm at room temperature) when the composition thereof has 2 for x, it contains a phosphate (PO.sub.4) group and, therefore, reacts readily with fused sodium which is one of the electrode materials of the sodium/sulfur storage battery.
Na.sub.4 Zr.sub.2 Si.sub.3 O.sub.12 (NZS), which has 3 for x and lacks the moiety PO.sub.4 of the material mentioned above, exhibits chemical stability to fused sodium and is a material with practical potential. Since this material has notably low conductivity (10.sup.-6 to 10.sup.-7 S/cm at room temperature) as compared with .beta.-alumina, however, it cannot be used in its unmodified form as a solid electrolyte for the sodium/sulfur storage battery.
The present inventors therefore pursued a study to find a means for enabling the NZS to retain its chemical stability and serve as a fully effective solid electrolyte for the sodium/sulfur storage battery. They consequently ascertained that a composite of NZS with titanium oxide acquires high conductivity without the loss of the chemical stability of the NZS. The present invention was accomplished as a result.